Hmm.
This might seem extreme but once upon a time, I loaded up my axe with a set of strings, and because I left them outside, they rusted...
"OMG, what do I do???"
Well I used metal polish (Autosol, to be exact), run up and down along the strings thoroughly, cleaned them up with string cleaner and viola! No more rust, and back to good, nice tone once again.
But only do this if you have at least 1 hour's worth of spare time which might otherwise be dedicated to playing your instrument. You have been warned.